"MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It’s gotta be telling us something."
No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.
It's not like MOND (or even dark matter) are even single theories with a fixed set of predictions. They are groups of competing ever evolving theories. I don't understand the need to believe one over the other as a predictive tool without some conclusive evidence. If you're not the astronomer who makes that measurement (or explains it), it just feels like galactic sports betting.
MOND successfully predicts many things we observe that DM fails to, or that requires special tuning for DM to fit observations (which is not prediction). DM actually has quite a poor predictive record, even if it can be made to fit observation.
MOND as a theory is probably not correct. It's not relativistic for a start, although I believe there are other modified gravity theories that are.
I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree).
Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?
> I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree).
> Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?
Pick any MOND theory you like, it all fails when it hits relativity, which is a theory with an embarrassment of riches of evidence in its favor, and can't be reconciled with relativity either. So while it explains galactic rotational speeds, it then fails to explain lots of other things, so it's a huge step backwards. It's the equivalent of saying Newtonian physics is wrong because it can't explain Mercury's precession, so let's go back to epicycles. To favor MOND we give up tremendously more than we gain.
MOND CAN explain things, if I have been interpreted to say it never predicts anything, that was never my intent. The problem is that MOND can't explain much else, so rather than making our theories simpler (which usually means we're in the right direction) it complicates things.
To make relativity work for the things MOND looks at, we only add 1 thing, WIMPs, particles we theorize but haven't seen. We've predicted lots of particle and found them, so this isn't a problem. chances we have missed a particle that turns out to be highly non-interactive? High. It took us ages to really solve the missing neutrino problem by discovering them, and we predicted the Higgs boson with high accuracy too. So this is a road we've been down before.
To make MOND work, we throw out a lot of theory, and we have nothing to replace it with under MOND. Chances all those other theories are wrong even though they work great? Low.
Do tell me if I'm completely wrong but isn't part of the problem there that you're simply expecting too much from MOND?
I mean a theory that is a modification of classical dynamics is not going to work well in a relativistic regime. That just means that somehow the classical limit ends up being slightly different from what we thought it was for reasons we don't know yet. I've always viewed MOND as more of an empirical law that we don't have a good explanation for yet. To me at least it seems obvious it will be deficient when you extrapolate too far and it can't replace more fundamental models.
That MOND can't handle relativity is not a direct reason to dismiss it. It would be a problem if relativity can't handle MOND (i.e. relativity cannot result in a MOND like theory) but I can't tell if that is the case. We could also dismiss MOND if it didn't explain anything, but as you said that's not quite the case either.
So yeah, dark matter seems more reasonable, but it's hard to completely ignore a theory seems to have more predictive power (even if we know it is flawed).
> Do tell me if I'm completely wrong but isn't part of the problem there that you're simply expecting too much from MOND?
No, and the reason is because MOND is a fundamentally flawed path. We're fairly sure unified theories are the way the universe works, MOND sticks out of that like a sore thumb. If MOND wants to explain one thing, then it has to fit everything else that effects or it's wrong. Really, for a theory to overturn a preferred one, it has to be better in some way. MOND isn't, it's explains few things than dark matter, and less well. At no point have I ever heard anyone state that MOND has a better predictive power than any other accepted theory, so I'm not sure what you're referring to there. In fact, that's the primary issue with MOND theories is that they ALL fail to meet dark matter with parity, none exceed it's predictive qualities.
For one thing MOND doesn't actually appear to eliminate the need for dark matter[1]. It gets rid of ~80% of the missing mass requirement, but not all of it. Whereas pure DM can just eliminate MOND.
Then you've got the "Bullet Cluster"[2] - where two colliding galaxies have had their observable and dark matter masses apparently separated. MOND can't explain this one without a lot of tweaking, but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't. The Bullet Cluster shows a galaxy shaped blob of gravitational lensing exactly where you'd expect it to be if a bunch of non-interacting matter had flown through each other, whereas the electromagnetic matter has interacted and re-shaped.
NGC 1052-DF2[3] and NGC 1052-DF4 are both ultra-diffuse galaxies which have no, or very little dark matter. That is, they appear to have normal galactic rotation curves fully explained by their observed visible mass. This works totally fine for DM existing (it's a problem for lambda-CDM though because it's not clear how they could've formed without dark matter, but I mean - we also don't yet know how black holes actually manage to ever merge either yet we do observe them too). This one always seems like a problem to me: MOND proposes a new universal principle of matter, then suddenly we have some matter where it's not doing that.
The theoretical problems[4] are somewhat beyond me, but they get well into issues with violating relativity and that's a big one: relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision. Build a better instrument, you can just dial in your precision and get the answer out ahead of time before you launch the satellite which is testing it. Also without careful adjustment you get violations of conservation of momentum (conversely, if MOND is real this would be handy because maybe it means we can reactionless spacedrives).
It's worth noting that none of this is implicitly fatal. lambda-CDM could be wrong, a MOND variant could be right. But a list of convenient things MOND explains easily doesn't escape the need to also include the things it can't - and appeals to the idea that DM is being "tweaked" to match observations unnaturally ignores the fact that MOND has to have the same thing done to it to fix within cosmology.
> Then you've got the "Bullet Cluster"[2] - where two colliding galaxies have had their observable and dark matter masses apparently separated. MOND can't explain this one without a lot of tweaking
The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution. All galactic clusters have issues in both MOND and LCDM, the bullet cluster was nothing new when it was discovered, it was just visually dramatic because they could image the gravitational lensing.
> but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't.
Actually LCDM can't explain the bullet cluster either:
As always, MOND and LCDM appear to just trade off one set of issues for other equally problematic set of issues. Neither is favoured very strongly by the sum of evidence. Physicists have just gotten in the habit of ignoring all of the problems with LCDM and consider even trivial problems with MOND to be fatal.
> relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision
Those precise tests do not extend to galactic scales, which is exactly where the problems appear. It would be nice if our existing theory worked across all scales, but that doesn't mean it must.
> "The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution."
If MOND requires "dark matter" to explain the bullet cluster, then what is the appeal of MOND?
Read the article this thread is about. Particle DM is not sufficient to explain all observations without extreme contortions that make MOND seem more reasonable. Also, MOND predictions made decades ago keep being validated. Why does this keep happening if there's nothing to MOND?
Finally, as I said, particle DM can't fully explain the Bullet Cluster either. The evidence is screaming in our faces that we need better thinking here.
It is one brand new paper. I am skeptical. Rotation curves are perfectly flat out to whatever arbitrary distance that they happen to be able to measure? I am very skeptical.
> "particle DM can't fully explain the Bullet Cluster either."
According to one scientist, who happens to be the same scientist claiming that particle DM cannot explain rotation curves. I will not check every claim, but the bullet cluster collision speed "problem" is readily explained in the reference in the wikipedia article: https://arxiv.org/abs/1410.7438
The broader point is that every time MOND has claimed to refute dark matter so far, the refutation has been refuted, so I will wait to see the outcome of this new claim.
I wasn't referring to the bullet cluster specifically, but this obsession with the bullet cluster is typical of the confirmation bias in this field: hyperfocus on what confirms bias and ignore the countervailing evidence. The past 30+ years have seen many "corrections" to get LCDM to fit observations it did not predict [1]. Clusters in general pose challenges to both MOND and LCDM for different reasons [2,3], but LCDM's typically get ignored and MOND's treated as a fatal blow. As I said, neither theory is fully satisfactory, but it's clear that research on these questions is fairly one-sided.
I personally hope that the solution to the handful or two of things that the LCDM model does not (currently) explain will be more interesting than a trivial tweak (which seems to have no physical motivation) to the gravity equation that MOND is.
Wikipedia's bullet cluster article is written in a very misleading way. The bullet cluster is not a hard problem for MOND since regular baryonic gas easily explains the lensing. Working out the bullet cluster with LambdaCDM actually took quite a bit longer because there were problems in the initial data set that took astronomers 10 years to work out. In short, the bullet cluster doesn't "disprove" MOND anymore than it "proves" LambdaCDM.
NGC 1052-DF2 is as big a problem for LambdaCDM as it is for MOND. LambdaCDM requires dark matter for galaxy formation. Which means either LambdaCDM is wrong about how galaxies form, or there was some event (of which we currently lack evidence) that removed it. As for MOND, it's possible that actually doing the math may show that this galaxy is not particularly weird. There's aren't many folks fluent in MOND, so work on these kinds of issues tends to lag their discovery.
As for MOND and cosmology, yeah, MOND sucks at cosmology. The fact that it can so easily explain so many galactic dynamics (far better than LambdaCDM) is really weird though. MOND isn't a great theory, but it's a really interesting model because it really highlights how poorly LambdaCDM predicts (not explains!) the galaxies we see today.
This is an apple's to oranges comparison. Lambda-CDM is a theory of all cosmology, including galaxy formation. The dark-matterless galaxies are a problem for Lambda CDM because they don't easily fall out of numerical simulation during formation. Supermassive black hole mergers also don't fall out of numerical simulation easily, but we still observe them.
They're not a problem for the concept of dark matter in general, since they have good agreement with the idea that the phenomenon creating dark matter appears to be a massive particle of some sort capable of being spatially dislocated from visible mass. So a theory proposing a pervasive but ill-described massive, mostly non-interacting particle may just be under-explored or the ramifications of the full scope of possible dark matter configurations (as pure mass) not explored.
It is a problem for MOND, which proposes that all matter is generating this effect which looks like dark matter (and then has an extremely poor ability to explain the rest of cosmology or work within the framework of general relativity).
The latest is, afaik, is called RelMOND, short for relativistic MOND. It describes the cosmos very similarly to the standard model, but with MOND as well. So slightly better matching observations than lambda CDM, but still imperfect. They plan to incorporate electromagnetism in future models.
No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.